1. Field of the Invention
The present invention generally relates to a heat-transfer tube or pipe equipped in a heat exchanger for use in an air-conditioning apparatus or the like, and more particularly to a heat exchanger tube preferably used for an air-conditioning apparatus using non-azeotropic coolant.
2. Prior Art
One conventional heat exchanger tube will be explained with reference to FIGS. 17 and 18. FIG. 17 is a perspective view showing a heat exchanger tube 1. In FIG. 17, heat exchanger tube 1 has an end being cut obliquely with respect to a center line 3 of heat exchanger tube 1. A plurality of grooves 2 are formed on an inside wall of heat exchanger tube 1.
FIG. 18 is a perspective view enlargedly showing a conventional groove configuration at a portion corresponding to "A" of FIG. 17. Ridge portion of the groove configuration comprises a top surface 4 and side surfaces 5. Between parallel two ridge portions, there is provided a flat bottom (recessed portion) 6.
Top surface 4 extends flatly in the longitudinal direction thereof. Opposed two side surfaces 5 are inclined with respect to bottom 6 at the same angle .beta..
FIG. 19 is a perspective view enlargedly showing another conventional groove configuration at a portion corresponding to "A" of FIG. 17, for example shown in Unexamined Japanese Patent Application No. HEI 3-189013, disclosed in 1991. Each protrusion, formed on an inside wall of heat exchanger tube, comprises a slant surface 7. A bottom comprises a slant surface 8 and a stepped portion 9.
However, if the former conventional groove configuration is adopted for a heat exchanger tube of the air-conditioning apparatus using non-azeotropic coolant, it will encounter the following problems. Non-azeotropic coolant has a difference between its boiling point and its dew point under the same pressure. When the difference between its boiling point and its dew point is approximately 5.degree. C., an inlet temperature at a vaporizer is decreased to -2.5.degree. C. under settings of an average vaporization temperature at 0.degree. C. The surface of fins near the inlet of the vaporizer will be bothered with icing of condensed water, deteriorating the ability of the heat exchanger.
To prevent such icing phenomenon, pressure loss in the heat exchanger tube is normally increased by changing the groove configuration in the heat exchanger tube, reducing the inner diameter of the heat exchanger tube, or reducing the number of fluid passages in the heat exchanger. Increase of pressure loss in the heat exchanger tube leads to an increase of inlet pressure and increase of inlet temperature.
However, to increase the pressure loss in the heat exchanger tube, using the former conventional groove configuration will undesirably increase the pressure loss in the condenser. Increase of pressure loss in the condenser leads to decrease of condensation temperature, deteriorating the condensation ability.
According to the latter conventional groove configuration, fluid in a vaporization phase flows in the direction of "B" while the fluid in a condensation phase flows in the direction of "C". Slant surface 7 acts to reduce the pressure loss in the condensation phase, however stepped portion 9 acts to increase the pressure loss in the condensation phase. In short, slant surface 7 and stepped portion 9 act oppositely in such a manner that they mutually cancel their effects. According to the latter conventional groove configuration, protrusions and recesses are formed by changing the pressure of rolling processing so as to form a protrusion by an amount excluded from a recess. In other words, a cross-sectional area normal to the center line of the heat exchanger tube is not changed regardless of formation of protrusions and recesses.